The present invention generally relates to the gasification of carbon-containing materials to produce fuel gases, and more particularly to a highly efficient gasification method which avoids problems caused by the formation of undesirable system byproducts.
Gasification is a process which generally involves the pyrolytic conversion of solid carbon-containing materials to gaseous products. Gasification is traditionally accomplished by the high temperature thermal breakdown of feedstock materials in the presence of steam, oxygen, air, and/or other suitable gases. Furthermore, gasification may involve either updraft, downdraft, crossdraft, fluid bed or entrained flow systems known in the art.
When carbon-containing materials are gasified, "fuel gas" is produced consisting of CO, CO.sub.2, H.sub.2, N.sub.2, H.sub.2 O, CH.sub.4, and other light hydrocarbons in varying proportions and amounts. Residual tar and oil materials are also produced as byproducts entrained in the pyrolysis gases. These materials are extremely viscous, and condense on piping and other equipment in the gasification system. They may also combine with char produced in the system to form layers of a solid organic residue which are extremely difficult to remove A promising method for removing the undesired tar/oil byproducts as described herein involves catalytic oxidation of the tars and oils However, when tar/oil destruction is accomplished using catalytic processes, carbon is deposited on the catalysts This ultimately deactivates the catalysts, rendering them ineffective.
Many attempts have been made to develop high efficiency gasification systems which minimize the problems described above. For example, U.S. Pat. No. 4,344,373 to Ishii et al discloses a gasification system including a fluidized bed pyrolysis reactor in which the endothermic decomposition of waste occurs, and a fluidized bed combustion reactor for the exothermic combustion of char, oils, and tar.
U.S. Pat. No. 4,135,885 to Wormser et al discloses a chemical reactor having a first upstream fluidized bed in combination with a second downstream fluidized bed. The upstream bed is designed to burn coal, while the downstream bed desulphurizes the gases produced from the burning coal.
Other gasification systems of interest are disclosed in U.S. Pat. Nos. 4,541,841 to Reinhardt; 4,300,915 to Schmidt et al; 4,028,068 to Kiener; 4,436,532 to Yamaguchi et al; 4,568,362 to Deglise et al; 4,555,249 to Leas; 4,372,755 to Tolman et al; 4,414,001 to Kunii; and 3,759,677 to White.
However, a need still exists for a highly efficient gasification system in which problems associated with undesired tar/oil formation and catalyst contamination are controlled. The present invention accomplishes these goals, and represents an advance in the art of gasification technology.